Rotary degassers and components therefor

ABSTRACT

Disclosed are degassers, couplings, impeller shafts and impellers for use in molten metal. One such coupling transfers gas into an impeller shaft, the coupling having a smooth, tapered internal surface to align with a corresponding surface on the impeller shaft and help prevent gas leakage and to assist in preventing damage to the impeller shaft. Improved impellers for shearing and mixing gas are also disclosed, as is a degasser including one or more of these components.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a Continuation Application ofU.S. patent application Ser. No. 14/923,296 (Now U.S. Pat. No.9,657,578), filed on Oct. 26, 2015, entitled “Rotary Degassers andComponents Therefor,” and invented by Paul V. Cooper, which is aContinuation Application of U.S. patent application Ser. No. 13/973,962,(Now U.S. Pat. No. 9,328,615), filed on Aug. 22, 2013, entitled “RotaryDegassers and Components Therefor, and invented by Paul V. Cooper, whichis a Divisional Application of U.S. patent application Ser. No.12/878,984, (Now U.S. Pat. No. 8,524,146), filed on Sep. 9, 2010,entitled “Rotary Degassers and Components Therefor,” and invented byPaul V. Cooper. Each of the foregoing disclosures of which that are notinconsistent with the present disclosure are incorporated herein byreference. U.S. patent application Ser. No. 12/878,984, (Now U.S. Pat.No. 8,524,146), also claims priority to U.S. Provisional Application No.61/240,981, filed on Sep. 9, 2009, entitled “Impeller and DegasserCouplings for Molten Metal Devices,” and invented by Paul V. Cooper. Thedrawings and pages 29-35 of Provisional Application No. 61/240,981 areincorporated herein by reference. U.S. patent application Ser. No.12/878,984, (Now U.S. Pat. No. 8,524,146), is also a continuation inpart of and claims priority to U.S. patent application Ser. No.12/853,255, (Now U.S. Pat. No. 8,535,603), entitled “Rotary Degasser andRotor Therefor,” filed on Aug. 9, 2010, and invented by Paul V. Cooperand which claims priority to U.S. Provisional Patent Application No.61/232,384 entitled “Rotary Degasser and Rotor Therefor,” filed on Aug.7, 2009. The disclosures of U.S. patent application Ser. No. 12/853,255and U.S. Provisional Patent Application Ser. No. 61/232,384 that are notinconsistent with the present disclosure are incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to couplings, impellers and rotary degassers usedin molten metal. One aspect of the invention is an impeller shaft foruse with an impeller shaft that transfers gas, wherein the couplingdecreases the possibility of impeller shaft breakage, gas leakage andmaintenance. Another aspect of the invention is an improved impeller forintroducing, and mixing gas with molten metal.

BACKGROUND OF THE INVENTION

As used herein, the term “molten metal” means any metal or combinationof metals in liquid form, such as aluminum, copper, iron, zinc, andalloys thereof. The term “gas” means any gas or combination of gases,including argon, nitrogen, chlorine, fluorine, Freon, and helium, whichmay be released into molten metal.

A reverbatory furnace is used to melt metal and retain the molten metalwhile the metal is in a molten state. The molten metal in the furnace issometimes called the molten metal bath. Reverbatory furnaces usuallyinclude a chamber for retaining a molten metal pump and that chamber issometimes referred to as the pump well.

Known pumps for pumping molten metal (also called “molten-metal pumps”)include a pump base (also called a “base”, “housing” or “casing”) and apump chamber (or “chamber” or “molten metal pump chamber”), which is anopen area formed within the pump base. Such pumps also include one ormore inlets in the pump base, an inlet being an opening to allow moltenmetal to enter the pump chamber.

A discharge is formed in the pump base and is a channel or conduit thatcommunicates with the molten metal pump chamber, and leads from the pumpchamber to the molten metal bath. A tangential discharge is a dischargeformed at a tangent to the pump chamber. The discharge may also beaxial, in which case the pump is called an axial pump. In an axial pumpthe pump chamber and discharge may be the essentially the same structure(or different areas of the same structure) since the molten metalentering the chamber is expelled directly through (usually directlyabove or below) the chamber.

A rotor, also called an impeller, is mounted in the pump chamber and isconnected to a drive shaft. The drive shaft is typically a motor shaftcoupled to a rotor shaft, wherein the motor shaft has two ends, one endbeing connected to a motor and the other end being coupled to the rotorshaft. The rotor shaft also has two ends, wherein one end is coupled tothe motor shaft and the other end is connected to the rotor. Often, therotor shaft is comprised of graphite, the motor shaft is comprised ofsteel, and the two are coupled by a coupling, which is usually comprisedof steel.

As the motor turns the drive shaft, the drive shaft turns the rotor andthe rotor pushes molten metal out of the pump chamber, through thedischarge, which may be an axial or tangential discharge, and into themolten metal bath. Most molten metal pumps are gravity fed, whereingravity forces molten metal through the inlet and into the pump chamberas the rotor pushes molten metal out of the pump chamber.

Molten metal pump casings and rotors usually, but not necessarily,employ a bearing system comprising ceramic rings wherein there are oneor more rings on the rotor that align with rings in the pump chambersuch as rings at the inlet (which is usually the opening in the housingat the top of the pump chamber and/or bottom of the pump chamber) whenthe rotor is placed in the pump chamber. The purpose of the bearingsystem is to reduce damage to the soft, graphite components,particularly the rotor and pump chamber wall, during pump operation. Aknown bearing system is described in U.S. Pat. No. 5,203,681 to Cooper,the disclosure of which is incorporated herein by reference. U.S. Pat.Nos. 5,951,243 and 6,093,000, each to Cooper, the disclosures of whichare incorporated herein by reference, disclose, respectively, bearingsthat may be used with molten metal pumps and rigid coupling designs anda monolithic rotor. U.S. Pat. No. 2,948,524 to Sweeney et al., U.S. Pat.No. 4,169,584 to Mangalick, and U.S. Pat. No. 6,123,523 to Cooper (thedisclosure of the aforementioned patent to Cooper is incorporated hereinby reference) also disclose molten metal pump designs. U.S. Pat. No.6,303,074 to Cooper, which is incorporated herein by reference,discloses a dual-flow rotor, wherein the rotor has at least one surfacethat pushes molten metal into the pump chamber.

The materials forming the molten metal pump components that contact themolten metal bath should remain relatively stable in the bath.Structural refractory materials, such as graphite or ceramics, that areresistant to disintegration by corrosive attack from the molten metalmay be used. As used herein “ceramics” or “ceramic” refers to anyoxidized metal (including silicon) or carbon-based material, excludinggraphite, capable of being used in the environment of a molten metalbath. “Graphite” means any type of graphite, whether or not chemicallytreated. Graphite is particularly suitable for being formed into pumpcomponents because it is (a) soft and relatively easy to machine, (b)not as brittle as ceramics and less prone to breakage, and (c) lessexpensive than ceramics.

Three basic types of pumps for pumping molten metal, such as moltenaluminum, are utilized: circulation pumps, transfer pumps andgas-release pumps. Circulation pumps are used to circulate the moltenmetal within a bath, thereby generally equalizing the temperature of themolten metal. Most often, circulation pumps are used in a reverbatoryfurnace having an external well. The well is usually an extension of acharging well where scrap metal is charged (i.e., added).

Transfer pumps are generally used to transfer molten metal from theexternal well of a reverbatory furnace to a different location such as alaunder, ladle, or another furnace. Examples of transfer pumps aredisclosed in U.S. Pat. No. 6,345,964 B 1 to Cooper, the disclosure ofwhich is incorporated herein by reference, and U.S. Pat. No. 5,203,681.

Gas-release pumps, such as gas-injection pumps, circulate molten metalwhile releasing a gas into the molten metal. In the purification ofmolten metals, particularly aluminum, it is frequently desired to removedissolved gases such as hydrogen, or dissolved metals, such asmagnesium, from the molten metal. As is known by those skilled in theart, the removing of dissolved gas is known as “degassing” while theremoval of magnesium is known as “demagging.” Gas-release pumps may beused for either of these purposes or for any other application for whichit is desirable to introduce gas into molten metal. Gas-release pumpsgenerally include a gas-transfer conduit having a first end that isconnected to a gas source and a second submerged in the molten metalbath. Gas is introduced into the first end of the gas-transfer conduitand is released from the second end into the molten metal. The gas maybe released downstream of the pump chamber into either the pumpdischarge or a metal-transfer conduit extending from the discharge, orinto a stream of molten metal exiting either the discharge or themetal-transfer conduit. Alternatively, gas may be released into the pumpchamber or upstream of the pump chamber at a position where it entersthe pump chamber. A system for releasing gas into a pump chamber isdisclosed in U.S. Pat. No. 6,123,523 to Cooper. Furthermore, gas may bereleased into a stream of molten metal passing through a discharge ormetal-transfer conduit wherein the position of a gas-release opening inthe metal-transfer conduit enables pressure from the molten metal streamto assist in drawing gas into the molten metal stream. Such a structureand method is disclosed in U.S. application Ser. No. 10/773,101 entitled“System for Releasing Gas into Molten Metal”, invented by Paul V.Cooper, and filed on Feb. 4, 2004, the disclosure of which isincorporated herein by reference.

Generally, a degasser (also called a rotary degasser) is used to removegaseous impurities from molten metal. A degasser typically includes (1)an impeller shaft having a first end, a second end and a passage (orconduit) therethrough for transferring gas, (2) an impeller (also calleda rotor), and (3) a drive source (which is typically a motor, such as apneumatic motor) for rotating the impeller shaft and the impeller. Thedegasser impeller shaft is normally part of a drive shaft that includesthe impeller shaft, a motor shaft and a coupling that couples the twoshafts together. Gas is introduced into the motor shaft through a rotaryunion. Thus, the first end of the impeller shaft is connected to thedrive source and to a gas source (preferably indirectly via the couplingand motor shaft). The second end of the impeller shaft is connected tothe impeller, usually by a threaded connection. The gas is released fromthe end of the impeller shaft submersed in the molten metal bath, whereit escapes under the impeller. Examples of rotary degassers aredisclosed in U.S. Pat. No. 4,898,367 entitled “Dispersing Gas IntoMolten Metal,” U.S. Pat. No. 5,678,807 entitled “Rotary Degassers,” andU.S. Pat. No. 6,689,310 to Cooper entitled “Molten Metal DegassingDevice and Impellers Therefore,” the respective disclosures of which areincorporated herein by reference.

Known coupling-to-impeller shaft connections are usually threaded, andgas can seep past the threaded connections, especially after the threadshave been worn after operation of the degasser, causing the graphitethreads of the impeller shaft to wear. The leaks waste gas, and ifcaustic gas such as chlorine is used, the gas can interact with nearbysteel causing the steel to oxidize as well as releasing the causticchlorine gas into the atmosphere creating an environmental hazard.

Another problem with conventional devices is that broken or wornimpeller shafts are difficult to remove. The impeller shafts, alsocalled “shafts,” “degasser shafts,” or “degasser impeller shafts,”herein, are usually formed of graphite, silicon carbide or somecombination thereof. The impeller shafts are typically connected to acoupling by a threaded connection wherein an internal cavity of a collarof the coupling is threaded and the external surface of the impellershaft is threaded, and threadingly received in the internal cavity ofthe coupling. Stress is placed on the impeller shaft as it rotates andthe shaft is weakened by the threads, so the impeller shaft tends toeventually break, and it typically breaks just below the coupling andthe end still threaded into the coupling must be chiseled out, which istime consuming.

Another known way to couple an impeller shaft to a steel motor driveshaft is by threadingly connecting it to a threaded projection extendingfrom the drive shaft. The projection comprises a threaded outer surfacethat is received in a threaded bore of the graphite impeller shaft. Inthis case, the single connection serves to both transfer torque to theimpeller shaft and to create a gas-tight seal with a threaded bore inthe impeller shaft. The impeller shaft is hollow, having an internalbore through which gas is transferred ultimately into the molten metalbath. Although this design allows for relatively easy removal of theimpeller shaft if the shaft breaks, the impeller shaft is not supportedor aligned by a coupling and the impeller shaft tends to wobble and thegraphite threads in the bore wear quickly. As the fit loosens, theimpeller shaft becomes more eccentric in its movement, i.e., it wobblesmore, and eventually breaks.

One attempt to solve the problems associated with coupling a graphiteshaft to a steel drive shaft is shown in U.S. Pat. No. 5,203,681 toCooper entitled “Submersible Molten Metal Pump.” This referencediscloses a two-piece clamp held in position by a through bolt. Shaftsretained by this clamp must include a cross axial bore to allow the boltto pass through the shaft. This structure would not be used by oneskilled in the art to couple a hollow shaft that functions as agas-transfer conduit because gas could leak from the holes formed aspart of the cross axial bore.

Further, many conventional devices do not adequately mix the gas beingintroduced into the molten metal. The gas can become trapped in a pocketwithin the impeller or rotor, or is otherwise not properly dispersedinto the molten metal. Additionally, if rotated too fast in order tomore thoroughly mix the gas and molten metal, “cavitation” can occur.Cavitation is when essentially a whirlpool is created that pulls airfrom the surface into the molten metal. This causes oxidation at thesurface of the bath and slag or other impurities may be formed.

SUMMARY OF THE INVENTION

In accordance with the invention a rotary degasser for introducing gasinto molten metal is disclosed. In one embodiment the degassercomprises: (1) an impeller (or degasser) shaft including a first end forconnecting to a coupling without the use of threads and an internalpassage that transfers gas; (2) an impeller coupled to a second end ofthe impeller shaft, wherein the impeller comprises: at least oneimpeller opening communicating with the impeller shaft passage, and theopening allows gas to escape into the molten metal under the impellerand enter at least one channel in the bottom of the impeller where it isdirected to at least one cavity, which is preferably defined in part bya curved side of the impeller; and (3) a coupling having a collar thatreceives the first end of the impeller shaft and retains it without athreaded connection. The impeller shaft is preferably connected to adrive source by the coupling and the drive source turns the impellershaft and the impeller. The impeller thereby displaces the molten metalwhile simultaneously gas is introduced into the molten metal through theopening.

An impeller of the invention may include at least a top surface and onecavity defined by a curved impeller side surface (or portion) juxtaposedan edge or other shearing structure. In the preferred embodiment, thedistance from the center of each curved impeller side surface is closerto a center of the impeller than the distance from each of the shearingstructures to the center of the impeller. One or more channels may beformed in the bottom surface of the impeller, wherein each channelextends from the opening in the bottom of the impeller to the center ofa respective cavity. There may be four channels, wherein each extends tothe center of a respective cavity. The impeller is preferablythreadingly received onto the second end of the impeller shaft.

In one embodiment a coupling configured to be connectable to an impellershaft preferably comprises an inner surface defining a smooth tapered,wall, and (2) at least one opening to receive a retention device, suchas a set screw. An impeller shaft according to the invention ispreferably not threadingly coupled to the coupling, so the coupling neednot include threads.

Another impeller according to the invention has at least one cavity in afirst vertical position and at least one cavity in a second verticalposition, wherein the second vertical position is above the firstvertical position. Preferably, there is a plurality of cavities in eachof the two vertical positions. Each cavity is juxtaposed an edge, orother shearing structure. The impeller includes a gas release openingfor allowing gas to escape under the impeller. At least some of the gasthen enters the first and/or second cavity(ies), where it is mixed withmolten metal as the rotor rotates. This impeller thus has two stages atwhich gas can be mixed with the molten metal.

Both the forgoing general description and the following detaileddescription are exemplary and explanatory only and are not restrictiveof the invention as claimed. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and together with the description serve toexplain the principals of the invention and not to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of a rotary degasseraccording to the invention.

FIG. 2 is a perspective view of an impeller and impeller shaft accordingto one embodiment of the present invention.

FIGS. 3A-3F are views of an alternate embodiment of an impeller andimpeller shaft according to the invention.

FIG. 4 depicts one embodiment of a coupling/impeller shaft connectionaccording to the invention.

FIGS. 5A-5D depicts alternative views of the coupling shown in FIG. 4.

FIG. 6 depicts an embodiment of the coupling/impeller shaft connectionas shown in FIG. 4, but showing the entire impeller shaft.

FIGS. 7A-7C depicts an embodiment of a set screw according to theinvention.

FIGS. 8A-8D depict an impeller shaft according to one embodiment of theinvention.

FIG. 9 depicts a plurality of rotary degassers according to theinvention separated by dividers in a molten metal bath.

FIG. 10 depicts the flow of molten metal and gas mixture utilizing arotary degasser according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to the present exemplaryembodiments of the invention, examples of which are illustrated in theaccompanying drawings. FIG. 1 depicts a gas-release device 10 accordingto the invention. Device 10 is adapted to operate in a molten metal bathB contained within a vessel 1. Vessel 1 includes a bottom 2 and sidewalls 3. Vessel 1 may have any suitable size, shape, and configuration.

The exemplary rotary degasser 10 includes an impeller shaft 100 (alsoshown are shafts 100′ and 100″), an impeller 200 and a coupling 20 forcoupling the impeller shaft to the motor shaft of a drive source (notshown). Impeller shaft 100, impeller 200, and each of the impellers usedin the practice of the invention, are preferably made of graphiteimpregnated with oxidation-resistant solution, although any materialcapable of being used in a molten metal bath, such as ceramic, ornon-impregnated graphite could be used. Oxidation and erosion treatmentsfor graphite parts are practiced commercially, and graphite so treatedcan be obtained from sources known to those skilled in the art.

The drive source can be any structure, system, or device capable ofrotating shaft 100 and impeller 200 and is preferably a pneumatic motoror electric motor, the respective structures of which are known to thoseskilled in the art. The drive source can be connected to impeller shaft100 in any suitable manner, but is preferably indirectly connected by amotor shaft that is connected to one end of coupling 20, whereas theother end (or collar) of coupling 20 is connected to a first end 102 ofthe impeller shaft 100. The motor shaft is preferably comprised ofsteel, comprises an inner passage for the transfer of gas, and ispreferably in communication with a rotary union, which releases gas froma gas source into the motor shaft, as is known by those skilled in theart. A typical rotary union is a rotary union of the type described inU.S. Pat. No. 6,123,523 to Cooper, filed Sep. 11, 1998, the disclosureof which from page 9, line 21 to page 10, line 23, and FIGS. 4 and 4D,are incorporated herein by reference.

As is illustrated in FIGS. 1, 4 and 6, shaft 100 comprises a first end102, a second end 104, a sidewall 106 and an inner passage 108 fortransferring gas. Shaft 100 may be a unitary structure or may be aplurality of pieces connected together. The purpose of shaft 100 is toconnect to an impeller to (1) rotate the impeller, and (2) transfer gasto the bottom surface of the impeller. Any structure capable ofperforming these functions can be used in conjunction with the presentinvention.

A preferred embodiment of the shaft 100 at end 102 is shown in FIGS. 4and 6. In this embodiment, first end 102 (which is received in coupling20) is tapered. It also comprises at least one groove 430 for receivingat least one retainer 411. In this exemplary embodiment, the groove 430in shaft 100 is helical and extends along the shaft 100 such thatpreferably two or more retainers 411 (and preferably as many as fouralthough any number may be used) can engage the groove at differentpositions to retain impeller shaft 100. As used herein with respect toend 102 of impeller 100, “groove” means any recess, indentation orstructure designed to receive a retainer.

The tapered portion 102A of end 102 of the impeller shaft 100 alignswith an internal, tapered portion 422 of coupling 200, as seen in FIGS.4, 5A and 5C. This alignment helps prevent gas from escaping between thetapered portion 102A of the impeller shaft 100 and the interior, taperedportion of the coupling 422. The groove 430 could extend onto thetapered portion 102A of the shaft 100, but it is preferred that thegroove does not extend onto portion 102A, since it may then weaken end102. The impeller shaft 100 is preferably threaded at second end 104 forbeing threadingly connected to impeller 200, although second end 104 maybe configured to couple with the impeller 200 in any suitable manner.

An embodiment of a coupling according to the present invention is shownin FIGS. 4-6. Coupling 20 vertically and rigidly couples a motor shaftto an impeller shaft, such as impeller shaft 100. Referring to FIG. 5,coupling 20 is preferably a one-piece coupling incorporating twocoupling members, first member 402 and second member 404. Member 402 canbe any structure designed to connect to and receive suitable drivingforce from a motor shaft. In the preferred embodiment, coupling 402 isdesigned to receive a motor shaft (which is preferably cylindricaland/or keyed), within the opening 440 formed in the member 402. Themotor shaft may be retained within the opening 440 in any suitablemanner, such as by using set screws 412 positioned in apertures 20A ofthe coupling 20 (not shown) spaced about the circumference of member402. In such a configuration, the set screws can be tightened againstmotor shaft to help retain shaft within the opening 440.

Second coupling member 404 (best seen in FIGS. 5A-5C) is configured toreceive the impeller shaft 100 through opening 410. The coupling member404 may engage the impeller shaft 100 in any suitable manner. In thepresent exemplary embodiment, one or more retainers 411 (which mayinclude bosses, bolt-retention devices, cap screws or set screws 412)engage the shaft 100 through apertures 20A.

In one embodiment, referring now to FIGS. 7A-7C, each of two retentiondevices 411 comprises a set screw 412 that aligns with an aperture 20Aformed in coupling member 404. Each set screw 412 is tightened to engagethe shaft 100, preferably by using a tool, such as an Allen wrench, inorder to secure shaft 100 in second coupling member 404. The threadedportion of each screw 412 preferably interfaces with correspondingthreads around the aperture 414. The portion of each screw 412 thatengages the impeller shaft 100 may be any size, shape, and configurationto retain the impeller shaft 100 within the coupling 20. In theexemplary embodiment depicted in FIGS. 7A-7C, the end of each set screw412 is sized, shaped, and configured to engage a groove 430 formed inthe surface of the impeller shaft 100.

When end 102 (as shown in FIGS. 8A-8D) is received in bore 404, taperedportion 103 of the impeller shaft 100 is received into the taperedportion 422. When these tapered, generally smooth surfaces align, theclose fit helps to prevent gas leakage and helps to center the shaft 100and reduce shaft vibration.

Turning now to FIGS. 2 and 3A-3E, embodiments of impeller 200 are shown.Impeller 200 is designed to displace a relatively large quantity ofmolten metal and thoroughly mix the gas being released into the moltenmetal. Therefore, impeller 200 can, at a slower speed (i.e., lowerrevolutions per minute (rpm)), mix the same amount of gas with moltenmetal as conventional devices operating at higher speeds. Impeller 200can preferably also operate at a higher speed at which it would mix moregas and molten metal than conventional devices operating at the samehigher speed.

By operating impeller 200 at a lower speed less stress is transmitted tothe moving components, which leads to longer component life, lessmaintenance and less downtime. Another advantage that may be realized byoperating the impeller at slower speeds is the elimination of a vortex.Some known devices must be operated at high speeds to achieve a desiredefficiency. This can create a vortex that draws air into the moltenmetal from the surface of bath B. The air can lead to metal ingots andfinished parts that have air pockets, which is undesirable and/or to theformation of dross. As shown by the arrows in FIG. 10, for example, theimpeller 200 of the present invention circulates gas throughout themolten metal bath B as it rotates without creating a vortex.

In one embodiment, impeller 200 comprises a top surface 202, sides 204,206, 208 (not shown) and 210 (not shown) corners 212, 214, 216 and 218,and a lower surface 220. Impeller 200 is preferably imperforate,rectangular and most preferably square in plan view, with sides 204,206, 208 and 210 being preferably equal in length. It also is possiblethat impeller 200 could be triangular, pentagonal, or otherwisepolygonal in plan view. A connector (not shown) is formed in top surface202. The connector is preferably a threaded bore that extends from topsurface 202 to lower surface 220 and terminates in gas-release opening223, though the impeller 200 can be connected to the shaft 100 in anysuitable manner.

This exemplary impeller 200 includes one or more cavities 224 defined inpart by each of curved sides 204, 206, 208 and 210, which are beneathupper surface 230. Each cavity 224 is preferably symmetrical about thecenter of its respective side (204, 206, 208, or 210), although one ormore of the cavities could be formed off center from its respectiveside. The cavities need not be identical to each other as long as gasescaping through the gas-release opening enters each cavity where it isultimately mixed with the molten metal entering the cavity. Theinvention could function with fewer than or more than four cavities 224.Additionally, the cavities may be formed in any portion of impeller 200,rather than being formed at 90-degree intervals by the sides (204, 206,208, or 210) as shown in FIG. 2. Additionally, a cavity may have anysuitable size, shape, and configuration.

In the present exemplary embodiment, each cavity preferably comprises anidentical structure, therefore only one cavity 224 shall be described.Cavity 224 is partially defined by concave side surface 204, wherein thedistance from the center of the curved surface 204 is closer to a centerof the impeller 200 than the distance from ends (212, 214) of the curvedsurface 204 to the center of the impeller 200. Cavity 224 is furtherdefined by upper surface 230. In the present exemplary embodiment,surface 230 of the impeller 200 is substantially flat and circular asviewed from the bottom of the impeller 200.

The impeller 200 may comprise one or more channels 225 in the bottomsurface 220 of the impeller 200. The channels 225 may be any size,shape, and configuration. In the present exemplary embodiment, thedevice comprises four channels 225, one that extends to in each of thefour side cavities.

The edges, such as edges 212, 214, 216 and 218, act as sheering surfacesto break apart gas bubbles into smaller bubbles as the rotor 200rotates. The impeller 200 is threadingly received onto the impellershaft. A lip 234 is formed between top wall 230 and top surface 202; lip234 preferably comprises a minimum width of one quarter of an inch.Lower surface 220 comprises edges 240 juxtaposed each of the recesses224. The impeller 200 is comprised of a heat resistant material such asgraphite or ceramic.

In one embodiment, the second end 104 of shaft 100 is preferablyconnected to impeller 200 by threading end 104 into a connector (notshown) on the impeller. If desired, shaft 100 could be connected toimpeller 200 by techniques other than a threaded connection, such as bybeing cemented, pinned or in any other suitable manner. The use ofcoarse threads (4 pitch, UNC) facilitates manufacture and assembly. Thethreads may be tapered.

Upon placing impeller 200 in molten metal bath B and releasing gasthrough passage 108, the gas will be released through gas-releaseopening 223 and at least some will flow outwardly through the channels225 in lower surface 220, and into each cavity.

As impeller 200 turns, the gas in each of cavities 224 mixes with themolten metal entering the cavity and this mixture is pushed outward fromimpeller 200. The released gas will also be sheared into smaller bubblesas they are struck by a shearing surface when rotor 200 rotates.

By using impeller 200, high volumes of gas can be mixed with the moltenmetal at relatively low impeller speeds. Unlike some conventionaldevices that do not have cavities, the gas cannot simply rise past theside of the impeller 200. Instead at least some of the gas enters thecavities 224 and is mixed with the molten metal.

An alternate, impeller 300 is shown in FIGS. 3A-3F. Impeller 300 ispreferably imperforate, formed from graphite and connected to, anddriven by, a shaft such as shaft 100 or shaft 100″. Impeller 300 furtherincludes a connective portion 304, which is preferably a threaded bore,but can be any structure capable of drivingly engaging shaft 100.

Impeller 300 includes two sets of cavities, wherein each set is at adifferent vertical position, that can capture gas and mix it with moltenmetal. Thus, impeller 300 is a two-stage impeller with respect to mixinggas and molten metal. Impeller 300 comprises a top surface 302, a bottomsurface 320, a first stage 360 and a second stage 370. First stage 360includes a plurality of cavities 362 wherein each cavity is juxtaposedby at least one edge, or other shearing structure, 362A.

Impeller 300 also has a second stage 370 that includes four sides 304,306, 308 and 310 four corners 312, 314, 316 and 318, and cavities 372.Impeller 300 is preferably imperforate, and rectangular (and mostpreferably square in plan view, with sides 304, 306, 308 and 310 beingpreferably equal in length). It also is possible that impeller 300 couldbe triangular, pentagonal, or otherwise polygonal in plan view. Aconnector 322 is formed in top surface 302. The connector is preferablya threaded bore that extends from top surface 302 to lower surface 320and terminates in gas-release opening 323, though any other suitableconnector may be used.

One or more cavities 372 are formed in part by sides 304, 306, 308 and310. Each cavity 372 is preferably symmetric about the center of itsrespective side, although one or more of the cavities could be formedoff center. Further, the invention could function with fewer than ormore than the cavities shown. Additionally, the cavities may be formedin any suitable portion of impeller 300 and may be of any suitable size,shape, or configuration.

An impeller 300 rotates, gas is released through opening 323 and atleast some of the gas enters the one or more cavities 362 and the one ormore cavities 372. The respective edges, or other shearing structures362A and 372A break the gas into smaller bubbles as rotor 300 rotatesthereby helping to disperse the gas into the molten metal.

Referring now to FIG. 9, any number of molten metal degassers of thepresent invention, as described above, may be employed in a molten metalbath B. In this exemplary embodiment, a plurality of degassers aredisposed in a molten metal bath B separated by dividers 910. Thedividers 910 may be made out of any suitable heat resistant material. Inthe preferred embodiment they are made from the same material as thewalls of the molten metal bath B. The dividers 910 may be any suitablesize, shape, and configuration and may partially or completely separateportions of the vessel 1. In one embodiment, the dividers 910 couple tothe top surface of the molten metal bath B; however, the dividers 910may couple to any wall of the vessel 1 such as a side wall 3, bottomsurface 2, or be suspended by an alternative support structure. Thedividers 910 may be coupled to the vessel 1 in any suitable manner, suchas by pressure fitting, cementing, clamping, welding, and/or beingformed as part of the vessel. The dividers 910 are may be positioned invarious locations within the vessel 1 or bath B. In some embodiments theplacement of the dividers 910 may travel the entire length of the vessel1 (they may be placed in any position) and may be repositioned into adifferent location with ease. The dividers 910 may divide each degasser,two degassers or more than two degassers. Any suitable number ofdividers 910 may be implemented. Multiple dividers 910 may be made ofdifferent materials, different dimensions and sizes, and may comprisedifferent openings for molten metal to pass through.

As shown in FIG. 9, there is preferably no gap between the sides of thedivider 910 and the side walls 3 of vessel 1, as the divider 910 runsthe entire width of the molten metal bath. In this embodiment, there isa gap between the bottom surface 906 of the molten metal bath B to thebottom most edge 904 of divider 910 to allow molten metal to flowbetween the chambers.

Having thus described some embodiments of the invention, othervariations and embodiments that do not depart from the spirit of theinvention will become apparent to those skilled in the art. The scope ofthe present invention is thus not limited to any particular embodiment,but is instead set forth in the appended claims and the legalequivalents thereof. Unless expressly stated in the written descriptionor claims, the steps of any method recited in the claims may beperformed in any order capable of yielding the desired result.

What is claimed is:
 1. A rotor shaft configured for use in a moltenmetal environment, the rotor shaft comprising one or more of graphiteand ceramic, and the rotor shaft being configured so that it cannot bethreadingly connected to a corresponding coupling, and is configured tobe received and retained in the corresponding coupling, which has nointernal threads, the rotor shaft comprising: (a) a first end that istapered and not threaded; (b) a non-tapered center portion that thatincludes at least one helical groove that is not threads, and that isconfigured to receive an end of a retainer, wherein the non-taperedcenter portion has a first side connected to the first end, and a secondside; (c) an outer surface connected to the second end side of thecenter portion, wherein the outer surface has no threads or grooves; and(d) a second end that is threaded and configured to connect to a rotor;wherein the first end of the shaft is configured to be received in thecorresponding coupling so that the first end of the shaft mates with aninner tapered portion of the corresponding coupling, and one or moreretainers are received in openings in the corresponding coupling suchthat each of the one or more retainers is positioned to be pressedagainst the at least one helical groove, in order to apply driving forcefrom the corresponding coupling to the shaft.
 2. The rotor shaft ofclaim 1, wherein the second end is connected to a rotor.
 3. The rotorshaft of claim 1 wherein the taper is at an angle between 20° and 45°.4. The rotor shaft of claim 1 that has a single helical groove.
 5. Therotor shaft of claim 2, wherein the taper is at an angle between 20° and45°.
 6. The rotor shaft of claim 2 that has a single helical groove. 7.The rotor shaft of claim 1 that is a unitary structure.
 8. The rotorshaft of claim 1 that further comprises an inner passage fortransferring gas.